Intertek AIM 

601 W. California Avenue, Sunnyvale, CA 94086, 408.745.7000 

16100 Cairnway Drive, Suite 310 , Houston, TX 77084, 832.593.0550 

 www.intertek.com/aptech 

The Increased Cost of 

Cycling Operations at 

Combined Cycle 

Power Plants 

TP203 

November 2012 

Prepared By 

Steven Lefton 

Nikhil Kumar 

Doug Hilleman 

Dwight Agan 

Intertek APTECH 

Sunnyvale, CA USA 

Presented at the International 

Conference on Cycling Plant 

Charlotte, North Carolina 

November 2012 
The Increased Cost of Cycling Operations at Combined Cycle Power Plants 

Presented at the International Conference on Cycling Plant 

Charlotte, North Carolina, November 2012 

Steven Lefton 

Nikhil Kumar 

Doug Hilleman 

Dwight Agan 

Intertek APTECH 

Sunnyvale, California 

1. Abstract 

The increased cycling operations (on/off and load following) at combined cycle power plants in the 

United States will increase the cycling-related production and maintenance costs at these plants. The 

increased cycling is primarily the result of additional variable generation of renewable wind and solar 

megawatts. Additional influences include market pricing for gas fuel and additional emission restrictions 

on existing coal-fired units. The resulting penetration of renewables into the North American grid is 

increasing the number of on-off and load cycling operations, which increase these combined cycle plants’ 

damage and, thus, maintenance costs. This will increase the need for spinning reserve megawatts, their 

costs, and the startup charges for putting combined cycle plants online. The number and the desire for 

faster online times for market conditions increases the damage severity of the gas turbine starts. They are 

increasing thermal transients with more rapid gas turbine acceleration and higher mass gas flows at higher 

exhaust temperatures that are reaching the heat recovery steam generators (HRSG). These factors impact 

the gas turbine and the HRSGs, as well as the balance of plant, plant water chemistry, and these 

ultimately reduce the overall plant reliability. The average starts on these gas turbines/combined cycle are 

increasing and the gas turbines/combined cycle run times are generally decreasing. Owners and 

operators, as well as regulators, need to be aware of the increased costs that occur. While capacity factors 

decrease, the production costs will also rise significantly due to cycling operations. This is a discussion 

of the negative impacts of cycling operations on a combined cycle power plant. In order to quantify these 

impacts, a new real-time costing code, Cost Advisor, which was developed by Intertek APTECH for a 

Original Equipment Manufacturer (OEM) of gas turbines to provide improved cycling cost information to 

owners and operators of combined cycle and cogeneration power plants, in order to provide improved 

cycling cost information to owners and operators of combined cycle and cogeneration power plants will 

be presented. An overview of the cost of cycling will also be presented and the lower-bound of the cost 

of cycling range of a limited number of these studies will be discussed and quantified. 

2. Introduction 

Every time a power plant is turned on and off, the gas turbine, HRSG or boiler, steam lines, steam 

turbines, and auxiliary components go through unavoidably large thermal and pressure stresses, which 

cause damage and incur additional future costs. This damage is made worse by the phenomenon that we 

call fatigue and creep-fatigue interaction damage. These cycling-related costs and damages are 
attributable to current startup, shutdown, and rapid load following, but they have not yet crystallized and 

occurred in newer combined cycle and fossil plants. Therefore, we have to estimate and compare them to 

similar combined cycle plants with longer histories, where we have estimated start/stop and load 

following costs, the so-called cycling costs. Typical start definitions vary with size, and damage increases 

associated with these operations as shown in Figure 1. 

Figure 1 

Definitions of Hot, Warm, and Cold Starts for Combined Cycle Plants. 

During the startup and shutdown of combined cycle power plants, there are operational constraints that 

are designed to avoid damage to the combustion turbine, steam turbine, and to a limited extent, the HRSG 

components. In the case of the HRSG, the startup and shutdown maximum permissible ramp rates are 

primarily determined by the gas turbine startup ramp rates that approach 3000°F/hour when measured on 

a minute basis during their occurrence, and these rapid rates significantly affect and reduce the 

components creep-fatigue life and increase the stresses in the HRSG and gas turbine hot section 

components. The ultimate result is that these combined cycle units incur significant increases in cost of 

maintenance and increase in forced outages due to cycling that would not impact a base loaded unit. 

In our work for National Renewable Energy Laboratory (NREL), Western Electricity Coordinating 

Council (WECC), and the Department of Energy on the impact of the wind and solar renewable 

penetration of the western United States grid, we observe a significant increase in starts as well as cycling 

cost on combined cycle power plants. Figure 2A represents the number of starts (multiplied by unit 

capacity) for combined cycle units, which start more with increasing renewable penetration. Figure 2B 

represents the total increase in cycling cost of combined cycle units in the western United States grid. 

These costs are developed based on Intertek APTECH’s lower bound cost submission to NREL and 

WECC. 

Generation Unit Cycling

Definitions

Load Cycling

LL1 Lowest Load at Which Design SH/RH Temperatures 

can be Maintained

LL2 Current “Advertised” Low Load

LL3 Lowest Load at Which the Unit can Remain On-Line (CC-60%)

AGC & Load 

cycling to 60%

1-2 hours 

offline

2-4 hours off line 4 hours or more

Fossil

Steam

CC
Figure 2A shows the number of starts (multiplied by unit capacity) for various renewable penetration scenarios.1

 

Figure 2B shows the estimated total cycling cost of combined cycle units for increasing penetration of renewables.2

,

3

This has also been reported by Strategic Power Systems in their data on the service hours per start of 

F-Class gas turbine machines at 33 hours per start in 2007, 38 hours per start in 2008, and increasing to 

52 hours per start in 2009. The 2009 data indicates increasing run times per start corresponding with a 

significant decrease in the gas fuel prices.4

 As a result of this expected increase in cycling of combined 

cycle power plants, we believe it is necessary to carefully examine the effect of cycling on the entire 

combined power plant and analyze how these cycles affect the costs due to increased starting and load 

following. We have observed that an estimated 30% renewable penetration on the grid will result in over 

a 100% increase in total cycling cost for combined cycle units alone (see Figure 2B). The bottom line is 

that many combined cycle plants will see a steady diet of increased starts, lower capacity factors, and 

increasing cycling costs. 

3. Effects of the Gas Turbine and Cycling on the Combined Cycle and Heat Recovery Steam 

 Generator 

HRSGs of many new combined cycle units have also been experiencing early cycling-related failures 

because the combined cycle plant has been designed primarily to attain high thermodynamic performance 

at base load ratings and not for cycling operations until around 2005 when the newer combined cycle 

plants have been moderately improved for cycling operations. However, these plants have yet to prove 

their abilities in long-term cycling services. While these combined cycle plants are designed at a lower 

 

1 The Western Wind and Solar Integration Study Phase 2, D. Lew, G. Brinkman, E. Ibanez, B.-M. Hodge, 

M. Hummon, A. Florita, and M. Heaney NREL

2 Power Plant Cycling Costs - N. Kumar, P. Besuner, S. Lefton, D. Agan, and D. Hilleman, Intertek APTECH 

Sunnyvale, California - NREL 

3

 Analysis of Cycling Costs in Western Wind and Solar Integration Study – GE Consulting 

4

 The Value of Reliability Data – A look back... A view of the future of an evolving generation mix. Salvatore A. 

Della Villa, Jr. Strategic Power Systems, Inc. 
cost to install than coal-fired units, they require more costly overhauls and repairs. Intertek APTECH has 

discussed these cycling-related issues affecting HRSGs in five Electric Power Research Institute (EPRI) 

reports and an American Society of Mechanical Engineers (ASME) paper: 

1. “Heat Recovery Steam Generator Tube Failure Manual,” Electric Power Research Institute, EPRI 

Technical Report 1004503, Final Report (November 2002). 

2. “Delivering High Reliability Heat Recovery Steam,” Electric Power Research Institute, EPRI 

Technical Report 1004240 (March 2003). 

3. “Interim Nondestructive Examination Guidelines for HRSG,” Electric Power Research Institute, 

EPRI Technical Report. 

4. “Evaluation and Correction of Thermal Transient and Cycle Chemistry Influenced HRSG 

Failures,” Electric Power Research Institute, EPRI Technical Report. 

5. “Effects of Flexible Operations on Turbines and Generators,” Electric Power Research Institute, 

EPRI Technical Report 100351, Final Report, Stephen Hesler, PM (December 2004). 

6. “Analysis of Cycling Impacts on Combined Cycle Heat Recovery Steam Generators and 

Evaluating Future Costs of Countermeasures to Reduce Impacts,” ASME Power 2008-60026, 

July 22-24, Orlando, Florida, USA. 

The effects of this additional cycling on the increases in wear and tear and associated increases in 

maintenance costs, equivalent forced outage rates, heat rates, and production costs are not well 

understood by utility decision makers, regional independent system operators, and public utility 

commissions. The interactions and tradeoffs of costs, reliability, and cycling effects are also not well 

known. In order to properly manage the assets of a power plant, a plant operator and asset manager has to 

obtain reasonably accurate estimates of the full costs of their typical operations, especially of its 

operational cycles and start types in order to bid the correct costs in an increasingly cycling environment. 

Only then can you maximize your plant’s value, reduce operational damage, recover your cycling costs 

with proper cost knowledge in bidding, and reduce both high-impact/low probability and frequent 

outages. 

4. Heat Recovery Steam Generator Damage and Failures 

HRSG tube failures are very costly due to cycling, as space is limited for repairs, and often it is cheaper to 

replace the section rather than cut your way into a section of the HRSG, make repairs, and weld your way 

out. Common cycling-dominated power plant failures occur mainly in the HRSG tubing and the HRSG 

casing. Superheater and reheater sections mainly suffer from thermal quenching transients due to water in 

headers from improperly sized/designed drains and from improperly operated/maintained attemperator 

spray systems that generally result in tube-to-header failures, as shown in Figure 3. 

Evaporator tube failures occur due to overheating and excessive deposition of iron and other chemical 

deposits that get formed and transported during cycling operations. Economizer tube failures occur from 

a variety of issues including: thermal fatigue due to quenching hot tubing, economizer steaming, external 

corrosion, and flow assisted corrosion, as shown in Figure 4. HRSG casing can be subjected to thermal 

fatigue and the effects of corrosion from air and fuel born corrosion, as well as improper water washing 

the gas turbine and carry over to the HRSG. 
Figure 3 

Superheater/Reheater Tube Failure/Cracking at Header. 

Figure 4 

Flow-Assisted Corrosion at HRSG Low Pressure Drum. 

4.1. HRSG Header Borehole Cracking and Drum Penetration Cracking 

The HRSG ramp rates are limited and often constrained by HRSG water chemistry conditions and the rate 

of heating and cooling of the thicker-wall pressure parts. However, when the temperature of metal, 

steam, or water rates change from 1500 to 3000°F/hour due to the gas turbine exhaust, the gas thermal 

ramp rate is extremely rapid and very significant when calculated using the temperatures as measured by 

their rates of change in less than 1-hour increments in which they actually occur. This rapid heating 
causes changes in temperature of the inner surface of the pressure part. The temperature differences 

develop through the thickness of the pressure part wall. This through-wall temperature gradient persists 

until stable steam or water temperature conditions are reestablished, and then time elapses for the inner 

surface temperature to stabilize. Further time is necessary to bring the entire wall thickness to a new 

stable temperature close to that of the inner surface. These transients drive the establishment of a 

temperature gradient through the wall of the component. The hotter inner surface wants to expand 

relative to the cooler bulk temperature of the header, but the material is restrained from doing so, causing 

the inner surface to develop a compressive thermal stress while the coolest parts experience a tensile 

stress. This process results in cracking and more of this type of cracking has been observed on heavily 

cycled units and is controlled by the transient and the creep-fatigue interaction properties of the 

components. 

Headers are heated and cooled predominantly by heat transfer at their inner wall surfaces, tube bore hole 

surfaces, and pipe nozzle penetrations. Steam drums are heated and cooled predominantly by heat 

transfer at their inner wall surfaces, as are downcomer/riser pipe/steam outlet pipe nozzle surfaces, and 

feedwater nozzle penetrations. These high stresses and cracks, especially during shutdown, are well 

described in many papers.5

Superheater and reheater attemperators have been documented as a routine cause of high temperature tube 

failures and header cracking from thermal shock damage as these are cycled in routine load following and 

on/off cyclic operation. 

4.2. HRSG Tube-to-Header Cracking in Economizer, Superheater, and Reheater 

Due to the rapid gas turbine temperature rise of the gas turbine exhaust into a cold or warm HRSG, there 

is very uneven heating of large tube banks at very rapid rates noted above, especially when compared to 

conventional fossil power plant heat-up rates, typically at much lower temperatures of 500 to 

1500°F/hour. The effects of cycling, and especially condensate that is not removed from these tube banks 

from the previous shutdown, make the uneven heating cause very high tube-to-tube temperature 

differences leading to differential expansion tube-to-header creaking and failures. These effects are very 

damaging and we have written extensively on the effects and the resulting tube-to-header failures, as 

shown in Figure 3, and as discussed in the ASME paper, “Analysis of Cycling Impacts on Combined 

Cycle Heat Recovery Steam Generators and Evaluating Future Costs of Countermeasures to Reduce 

Impacts.” These failures are in part because of the inferior creep-fatigue properties, such as those of T91 

tubing, generally found in the high temperature sections of these HRSG units (Figure 5). 

5. Effect of Cycling on GT Operating Costs 

During every startup and shutdown and in load cycling to minimum load of the gas turbine, the hot-gaspath components sustain significant thermal cycling. These thermal cycles frequently shorten the lives of 

these parts and also reduce the inspection and overhaul intervals required for the engine as compared with 

an engine running continuous duty (base load). This shortening of the life of the parts and overhaul 

intervals increase operating costs in two ways. First, more parts have to be replaced during the overhauls 

and inspections, so-called parts fall out, which increase the costs of the overhaul. Often at the hot gas 

path inspection, parts are found to be damaged and are not repairable. For example, when hot section 

parts crack (thermal barrier coating crack) due to cycling, the cracks often proceeds into the base metal 

(see Figure 6). Figure 7 is an example that shows the effects of gas turbine blade coating cracking that 

 

5

 “Optimization of F-Class Combined Cycle Unit Starts and Shutdowns Based on HRSG High Pressure Superheater 

Header Tubehole Cracking Avoidance Constraints”, M. Pearson of J. Michael Pearson and Associates Co Ltd., 

J. Grover and S. Paterson of Aptech Engineering Services, R. Anderson of Competitive Power Resources, and 

B. Dooley of Structural Integrity Associates.
proceeds into the base metal. The part is not repairable and must be replaced at a cost not expected or 

generally included in most long-term service agreements. We have seen these very significant costs for 

the replacement of 10 to nearly 100% (depending on the number of cycles, the thermal barrier coatings, 

and the gas turbine manufacturer) of the blades and vanes in the first three rows of the gas turbine hot 

section and the combustor. In addition, there are the compressor thrusting and compressor-to-case rubs 

that have increased the number of total gas turbine catastrophic failures that has been shown to be startrelated. Secondly, because the costs of these replacement parts and total overhauls are disbursed earlier in 

time, there is an additional cost due to the time value of money. 

Reference Intertek APTECH analysis of Yukio, Takahashi’s “Follow Up Investigation on Creep-fatigue Evaluation of Various Alloys” 

Figure 5 

Creep-fatigue Interaction Curve. 

Figure 6 

Gas Turbine First Stage Blade Thermal Barrier Coating Cracking Extending into Base Platform.
Figure 7 

Cross-section View of Pressure Side Airfoil Cracking of Surface Coating Extending into Vane Base Metal of a Gas 

Turbine Blade. 

6. Steam Turbine and Generator Effects of Cycling 

Figure 8 shows severe turbine nozzle block fatigue cracking and case cracking from a heavily cycled 

steam turbine. In order to predict damage in a steam turbine, gas turbine, or HRSG, known damage 

mechanisms must be correlated to temperatures and pressures in real-time and alarmed for the operators. 

The correlation of high pressure rotor bore surface temperatures and their ramp rates to thermal stresses 

are shown in Figure 9. Lacking a detailed stress analysis model, we have applied a statistical model to 

thermal transient data stress output from the manufacturer’s on-board turbine thermal stress analyzer. 

Our stress model had a 0.97 correlation coefficient and, thus, a nearly perfect fit to actual plant data that 

shows increased stresses with increased megawatt ramp rates. Intertek APTECH then combined the 

temperature-to-stress statistical model with its other cost models developed during a cost of cycling 

project. This allowed us to estimate the effects of changing thermal ramp rates on damage and, 

ultimately, on costs. More details are reported in the EPRI report “The Effects of Flexible Operations on 

Turbines and Generators.” 
Figure 8 

Actual Nozzle Block Cracking. 

Figure 9 

Large Turbine Rotor Stress Shown as a Function of Megawatt Ramp Rate. 
7. Effects of increased cycling on the Generator 

Increased generator failures and insurance claims have been noted and generators which were previously 

not a concern are now reported to be the number three in insurance damage claims.6

 With the increased 

starts, we have seen an increase in the generator lead failure, hydrogen leakage, seal oil in the generator 

stator, and the need to rewedge and rewind generators in cycling duty. Lube and seal oil ingress into 

generators cause wedges and windings to become loose as stator iron gets very hot with oil pluggage, 

causing reduced localized cooling of the stator core. The hotter core iron then expands causing areas of 

the core iron to loosen its tight compaction. Pieces of the loose stator core iron have been dislodged, 

eroded, and in some areas fatigued-off, causing these small pieces to act like magnetic termites and short 

windings. All these mechanisms cause high-impact/low-probability events to become more common in 

cycling units with long outages and high repair costs. 

8. Balance-of-Plant Effects of Cycling 

In addition, there is significant damage in the balance of plant that is caused by 1) cycling-related 

chemical upsets in feedwater and boiler water, 2) equipment corrosion during offline and startup periods, 

3) excessive deposits, buildup, and under-deposit corrosion in the boiler, e.g., corrosion transport from 

air-cooled condensers to the HRSG, and 4) other transient behavior causing damage during starts, load 

cycling, and shutdowns. 

We observe more switchgear, transformer, and large motor failures due to cycling operations. The impact 

of cycling on NOx control system and ammonia injection grids causes ammonium bisulfate to participate 

out on the HRSG tubing, causing more starts and offline cleaning to remove the deposits. 

Piping systems valves and check valves are particularly susceptible to cracking and failure when multiple 

unit 2:1, 3:1 combined cycle plants with multiple HRSGs are started up, and cooler steam is blended into 

an operating hot piping system. Piping systems need to be subjected to transient stress analysis and 

brought up to much higher standards to include these transient effects. All of these component failures 

cause increased costs. 

9. Top-Down Method for Estimating Cycling Costs 

Intertek APTECH has found that accurate estimates of total unit damage costs can be derived using a 

regression analysis of historical unit damage with historical costs and equivalent forced outages, along 

with component-specific data that indicate the breakdown of cycling costs among various cycle types 

(e.g., hot, warm, and cold starts, load follows). Intertek APTECH’s damage model is intended to provide 

information on the cycling-related damage for the entire unit. It is founded on physical models and uses 

plant temperature and other real-time thermocouple data (we call this “signature data”) to provide crossvalidation and calibration relative to the megawatt changes, but requires only hourly megawatt 

operational data to estimate damage. Relying solely on hourly megawatt unit load data is an inherent 

advantage due to the fact that these types of data are more readily available. In addition, hourly megawatt 

data provide an accurate history of past unit operations. Our damage model accounts for creep damage, 

fatigue damage, erosion, corrosion, and all other types of damage that are known to occur in most 

combined cycle power plants. 

Intertek APTECH’s damage model validation process has included the assessment of key components 

with finite element analysis and creep/fatigue analysis methods to determine remaining useful component 

 

6 Reference Combined Cycle Journal. 
life, augmented by information from thousands of laboratory failure and stress analyses of power plant 

components. These life analyses of key high cycling cost components are statistically calibrated to the 

failure history of these components. Then all damage is calibrated to actual plant costs and these costs 

must back predict or back cast historical costs to be validated. Traditional, uncalibrated engineering 

fatigue and creep analyses, however ambitious and expensive, are rarely useful and are often misleading 

in predicting cycling costs. 

Real-time, minute-by-minute thermocouple data (or “cycling signature data”) provide information about 

the damage mechanisms and accumulation rates that may occur as a result of cycling operations. The 

signature data are used to calibrate, revise, and/or confirm the results of the general damage and cost 

models described above. This is done by using the signature data as input to analytical models, as well as 

to qualitative “expert opinion reviews” for the evaluation of cycling-related damage during load 

transients. This helps to calibrate and confirm the susceptibility of a given unit to cycling-related damage, 

as reflected in capital and maintenance spending and cost per cycle calculated using the top-down and 

bottom-up regression techniques. 

Critical components for which detailed plant signature data are analyzed include the steam drums, water 

wall/evaporator tubing, superheater and reheater tubing and headers, economizer inlet, and startup system 

components, as well as turbine/generator-related components, such as valves, cases, generator windings, 

and steam chests. The maximum temperature ramp rate and the overall range of temperature change 

experienced by a component during the transient are key indicators of cycling-related creep and fatigue 

damage. These parameters are used to quantify the severity of each unit’s load, startup, and shutdown 

transients. The signature data are also used in evaluating and troubleshooting each unit’s cycling 

operations, leading to recommendations for temperature ramp rate limits, e.g., superheater ramp rates, 

economizer ramp rates in startup and during shutdown/cooling, and other operational improvements 

aimed at minimizing damage, maximizing the asset’s life and reliability, and reducing maintenance costs. 

Intertek APTECH uses a statistical regression model to develop best estimates of the total combined cycle 

equipment damage costs due to typical cycling, which include incremental equivalent forced outage rates, 

capital, and maintenance costs. These costs are first estimated for an equivalent hot start used to describe 

each cycle type, and then estimated for the typical cycles experienced by the specific units studied based 

on how they are operated. Figure 10 shows the results of this type of regression analysis typically run 

with and without equivalent forced outage costs to separate the forced outage effects. This, and our other 

correlations of starts to plant outages, confirm our assertions that cycling causes increased forced outages. 
Figure 10 

Best Estimate Forced Outage, Maintenance, Operation, and Capital. 

10. Bottom-Up Method for Estimating Cycling Costs 

The “top-down” method described above provides a measure of total cost of cycling based on historical 

spending of the whole plant without regard for specific components. The “bottom-up” method seeks the 

relative distribution of those costs as a means of apportioning the total costs and providing a sound 

methodology for evaluating potential countermeasures. By identifying the major costs of damage to 

components that currently contribute to the wear-and-tear elements of cycling costs, the calculated cost of 

cycling can also be adjusted in the future. Improvements identified reflect improvements made through 

identification of problem components with high costs and forced outage rates. Root cause analysis, 

hardware fixes, and changes in operating practices, component replacements, and upgrades can be 

implemented. The cycling costs estimated using the bottom-up method also serve as a “sanity check” on 

the unit-wide costs derived in the top-down method. When we also perform the bottom-up analysis, it is 

generally within 5% to 10% of the costs derived from the top-down method, which provides validation of 

our top-down methodology. Figure 11 gives an example of the distribution of cycling damage costs 

among the major plant systems of a combined cycle unit. 

Best Estimate of Smoothed Plant X Units 2, 3, 4, Gas Turbine Portion of the Combined Cycle Plant 

including Forced Outage, Maintenance, Operation and Capital Costs

$0K

$5,000K

$10,000K

$15,000K

$20,000K

$25,000K

$30,000K

$35,000K

$40,000K

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Year

Annual Outage, Maintenance, Operation 

and Capital Costs (in 2009 currency)

Smoothed Annual Candidate Costs in Year 2009 Dollars Best Fit of Annual Costs (Results in $XX per Equivalent Hot Start and a COV=5%)

Adjusted for major overhauls and assumes 474 EHS per year for cycling the three gas turbines in the future 
Figure 11 

Total Operations and Maintenance Costs for Combined Cycle. 

11. Results of the Cycling Costs 

The overall range of cycling costs compared with commonly assumed costs is shown in Figure 12, which 

includes all cycle types of hot, warm, and cold starts for the three unit types of small drum, large 

supercritical, and gas turbines. The unit’s specific analysis results depend heavily on the regression 

analysis of the costs versus cycles and the unit signature data during cyclic operations, including the range 

of all load changes. The increased incremental costs that are attributed to cycling are broken down into 

the following categories: 

1. Increases in maintenance and overhaul capital expenditures 

2. Forced outage effects, including forced outage time, replacement energy, and capacity 

3. Cost of increased unit heat rate, both long-term efficiency, and while at low/variable loads 

4. Cost of startup fuel, auxiliary power, chemicals, and extra startup manpower 

5. Long-term generation capacity cost increases due to unit life shortening 

Figure 11 gives an example of the breakdown of maintenance and overhaul cycling costs for a combined 

cycle power plant. Cost ranges of the lower-bound of studies that we have performed are significant. We 

published the lower-bound range of cost for combined cycle plants in the publication for NREL.7

 

 

7

 Impact of Wind and Solar on Emissions and Wear and Tear of Fossil-Fueled Generators, D. Lew, G. Brinkman, 

N. Kumar, P. Besuner, D. Agan, and S. Lefton. 
Figure 12 

Typical Cycling Costs. 

12. COSTCOM Methods Properly Predict Future Damage 

Intertek APTECH’s methodology, first developed in 1989 for fossil power plants, correlates plant 

operations with past damage and then calibrates real component costs to these damages. We make the 

effort to verify that the cost models accurately “predict” past costs so that they can quantify and predict 

future costs more accurately. When possible, we estimate cycling cost using at least two different 

methods with independent investigators (e.g., top-down regression methods vs. bottom-up cost 

accounting and in plant studies). The major combined cycle plant components are broken down by costs 

in Figure 11 and separate damage models are developed from analysis of plant signature data collected 

from the plant data acquisition systems. Cost calibrations are executed in all of these major component 

and cost areas. 

12.1. Description of Real Time Operations 

The developed code COSTCOM and our combined cycle Cost Advisor developed in 2012 for an OEM’s 

combined cycle power plants, is a real-time software system that allows power plant operators and 

managers to see the comprehensive costs of their cycling operation within 1-minute of the current 

operation as shown in Figures 13 and 14). It is a PC-based program which gathers data from the existing 

plant information system for through-the-plant network. The system includes three modules: 1) an engine 

that calculates the results based on the plant inputs of some 15-20 variables in real time, 2) a viewer that 

displays the real time and historical results, and 3) an administrator which is used for changing the display 

settings, such as the alarm level or localization settings for language, local currency, units of measure, 

temperature/ramp rate limits in green ( normal), yellow (caution), and red (do not exceed).8

 

8

 Cost Advisor and COSTCOM User Manual. 

“Quantify True Unit Cost Per Cycle”

Estimated Cycling Costs Often Wrong

Small Drum

Large 

Supercritical

GT Simple Cycle

GT Comb. Cycle

Unit Type

$5,000

$10,000

$100

$200

Typical Industry Value

(without consideration of true costs)

$3,000 - $100,000

$15,000 - $500,000

$300 - $80,000

$15,000 - $150,000

Potential Range

of Total Cost
Figure 13 

Combined Cycle Cost Advisor Main Screen. 

Figure 14 

Cost Advisor Cost Screen Page 2 Showing a Start and its Cost in Real Time. 
Based on the key damage process indicators (DPI) for equipment health showing temperatures, pressures, 

flow rates, stress levels, and power outputs from the critical plant components (e.g., gas turbines, HRSGs, 

and steam turbine generators), it classifies the current cycling transient type that is occurring and the 

damage and other costs being incurred during that transient. The relationship between the measured key 

variables and the damage and costs are determined by codification of an extensive cost of cycling study of 

the plant using the methodologies described above. Once that relationship is determined, certain 

parameters in the computational “engine” are customized for that unit so it can accurately determine the 

damage and cost of current operation from the key DPI being monitored in real time. The basic principle 

is that given the current plant design, past cycling damage at past cycling rates of change have given past 

historical costs and the damage models must be calibrated via multivariable regression analyses to these 

past costs in order to predict future damage and costs. 

COSTCOM also shows the level of overall plant damage costs per day being incurred during a cycling (or 

any other) operation. From this, the operator can determine if the unit can be “pushed” to get online and 

produce megawatts faster (when the costs being incurred are low and the dispatch “rewards” are high) or 

if the unit’s ramp rates should be reduced because damage levels are too high relative to marketplace 

rewards. For example, the main screen first provides alerts to the operator with colored bar displays to 

indicate high temperatures and high rate of change of DPI, such as megawatt, stresses, temperatures, and 

ramp rates in normal (green), normal (yellow caution), and alarming (red do not exceed) levels for the 

overall unit damage being incurred. Secondly, it shows the responsible key measured variables 

(temperatures, ramp rates, flows, and stress levels) for the high damage and real time costs. It can also 

give advance warning of damage and allow an operator to avoid a damaging transients and it encourages 

the operators to stay in the green box of normal temperatures and ramp rates. In studies of combined 

cycle units, we have had operators reduce the cycling damage as measured by signature data as much as 

30-50% during starts and trips/shutdowns by revised procedures and minor changes to plant hardware. 

This will have the effect on costs and damage rates in the future which will also be reduced by the 

30-50% reduction in damage per cycle. 

13. Conclusion 

Combined cycle plants will bear an increased burden of cycling operation and costs due to the increase in 

wind and solar integration into the grid. Thus, combined cycle plants not only need to know these costs, 

but also control their cycling costs as well as recover them in the electricity markets. These costs 

dominate the typical plants variable cost of operations and maintenance that should be broken down into 

cycling operations and maintenance and base loaded operations and maintenance. As part of the 

developments we have performed and published in “Impacts of Renewable Generation on Fossil Fuel 

Unit Cycling: Costs and Emissions”9

, we have presented (Figure 15) the lower-bound cost from our 

analysis of a large number of combined cycle units’ cycling costs. We believe most plants built before 

2005 far exceed these lower-bound costs, and plants built after 2005 are still in the honeymoon stage with 

low accumulated damage and may still exhibit similar lower-bound and beyond the lower-bound with 

significantly elevated cycling costs in the future. 

 

9

 G. Brinkman, D. Lew, National Renewable Energy Laboratory, N. Kumar, S. Lefton, Intertek APTECH, 

Venkataraman, G. Jordan, GE Energy Schenectady. 
Figure 15 

Lower-bound Hot, Warm, and Cold Starts, Maintenance and Capital Cost per Megawatt Capacity (Including 

Outliers). 
601 West California Avenue 

Sunnyvale, CA 94086 

408.745.7000 Fax 408.734.0445 

Mailing: PO Box 3440 

Sunnyvale, CA 94088-3440 

16100 Cairnway Drive, Suite 310 

Houston, TX 77084 

Phone: 832.593.0550 

Fax: 832.593.0551 

Toll Free: 800.568.3201

Intertek AIM 

139, 11215 Jasper Avenue 

Edmonton, AB T5K 0L5 

Canada 

Phone: 780.669.2869 

Fax: 780.669.2509 

Offices 

www.intertek.com/aptech